They only test the DNA for Chromosome 21 for Down Syndrome. Not even 13, 18, because of the lower accuracy. But if there is mutant to 13, 18, the physical appearance should be different enough to tell by Ultrasound, according to the genetic counselor we talked to.
As a matter of fact, I don't care much about gender, as long as s/he is healthy. I do want to know though, but I'm patient enough to wait till week 18 for teh big Ultrasound test.